Additives are used in polymeric materials for a large number of processing and end product purposes. The additives employed to treat polymeric materials generally include antioxidants, processing aids, slip agents, antiblocking agents, antistatic agents, blowing agents, lubricants, UV stabilizers, coupling agents, colorants, flame retardants, and metal deactivators.
The purpose of the various methods of introducing additives to polymeric materials is to provide sufficiently intimate contact of additive and polymeric material such that the additives perform their intended purpose. Other considerations that are important in evaluating the various methods of contacting additive and polymeric material include capital requirements, heat history subjected to polymer, degradation of heat sensitive additives, accurate measured addition of additives, versatility in meeting different additive package needs, range of acceptable additives in process, effect of process on downstream processing of polymer, environmental impact, and finally the addition system packaging, shipping, storage, inventory requirements, and convenience.
Known methods of introducing additives to polymeric particles include dry blending the materials, melting, and compounding the melted blend with extruders and pelletizing or powdering to the desired physical form.
Another method of introducing additives to polymeric particles is to contact such particles with additive at the extruder hopper during end use processing. Additives such as colorants, slip agents, processing aids, blowing agents, and others are introduced to virgin polymeric particles at this stage usually in concentrate form. In many instances, difficulty is encountered in metering the exact amounts of additive concentrate necessary to do a specific job. This is especially true for additives such as processing aids and external lubricants which are used at very low levels and usually cannot be added in a concentrate form.
Some polymers are presently being manufactured with technology that does not lend itself to such techniques as melt compounding and pelletizing. Many polymers such as high density polyethylene, linear low density polyethylene, and polypropylene emerge from the polymerization reactor in a dry granular form, i.e., in a form similar to that of a fluidized bed system. Presently, additives for these polymers must be introduced by melting, compounding, and then pelletizing. This extra step increases the cost of such polymer manufacturing operations and can adversely affect the properties of such polymers.
Another method of coating polymeric particles with additives is disclosed in Japanese Patent 56-021815. This patent teaches contacting polypropylene granules with a dispersion of additives in a solvent, followed by removal of the solvent. While some stabilization is thereby imparted to the coated polypropylene granules, the treated pellets have severe static electricity problems when subsequently processed, such as for example, by extrusion.
Some methods of coating polymeric materials with additives include the addition of the additive directly into the polymerization reactor such as disclosed in U.S. Pat. No. 4,853,426 and U.S. Pat. No. 4,879,141. These methods of mixing the polymeric materials with additive are not possible when using heat sensitive additives or additives with incompatible melting points. Additionally, these methods are not very versatile in that the polymer must be coated during production of the entire output of the reactor for the fixed reaction time.
Improved methods of contacting additives with polymeric materials are by way of the application of an aqueous emulsion system such as disclosed in U.S. Pat. No. 4,975,120. These aqueous emulsions or dispersions can be applied by spray, wipe, dip systems, or the like, to coat the polymeric particles before their normal drying or degassing operations.
While the latter application system represents an advance in the state of the art by replacing the need for organic solvents with aqueous-based application systems, the preparation of an emulsion requires special equipment and the emulsion needs to remain stable until use. The main disadvantage of this aqueous application system is that it contains substantial quantities of water. The presence of this water requires the shipping of a large volume to receive the desired amount of additives. The presence of water also requires special handling and shipping of the aqueous emulsions or dispersions. For example, these emulsions or dispersions tend to freeze when exposed to extremely cold temperatures. Unfortunately, merely heating the frozen emulsion or suspension does not always result in the re-formation of stable emulsions or dispersions. Thus, exposure to temperature extremes and long term storage can cause problems in processing these aqueous emulsions or dispersions when it is attempted to apply these additive systems to polymer particles.
In order to overcome the above-described limitations of the prior art, it would be desirable to prepare additive systems for the treatment of polymer particles wherein the additive systems can be easily handled and transported, yet are readily water dispersible to produce aqueous application systems useful for the treatment of polymer particles.